1. Field of the Invention
This invention relates to a lightproof cassette for holding a roll of light-sensitive web material, the cassette having an outlet slit oriented parallel to the axis of the roll, the slit being formed between two lips that are provided in particular with a lining preventing light entry.
2. Description of Related Art
Cassettes that are intended to be light-tight are known in many embodiments. They are used to hold rolls of photographic web material, either film or paper, for various uses, such as portrait photography, photo typesetting, x-ray photography and the like.
Large format cassettes in particular create difficulties in keeping the slit necessary for the web outlet lightproof over the entire width. Under mechanical stress, such as occurs when the web is pulled out, the slit area can be deformed resulting in a wider opening and entry of light. If the slit is narrowed, the forces necessary to pull the web out become too high. This is also particularly pertinent, because cassettes are prepared from synthetic resin, such as polyvinyl chloride, polystyrene, acrylonitrile/butadiene/stryene copolymer or polyethylene, and for cost reasons, can be obtained only with limited wall thicknesses. This disadvantage also appears if the lips are provided with, as is customary, a lining, such as velvet, felt or soft plastic, to prevent light entry.
In one cassette disclosed in German Patent 2,615,615, the lips are positioned at a relatively acute angle, in particular 90.degree., on the inner side of the cassette housing to improve shape stability and are provided with a reverse curved edge on their free ends. However, this restricts significantly flexibility in the placement of the outlet slit.
Another construction disclosed in Eurpoean OS 153,505, has, instead of stiff lips, a number of interlocking elastic lips. This is significantly more expensive.
In another construction disclosed in German OS 2,950,624, a convex (with respect to the slit) lip is in positive tension contact with a concave lip and is separated therefrom when the web is pulled out. This does indeed impart good light exclusion ability, but it increases the force required to unwind the roll.
Although each of these disclosed cassettes consist of a cuboid or cylindrical mid-section, which has lips and two lightproof, side-wall closures joined to it, there are also on the market cassettes consisting of two injection-molded half shells with two projecting lips. The half shells are joined together along their edges. Problems similar to those already disclosed occur at their outlet slit.
It is believed desirable to produce a lightproof cassette with improved light exclusion ability without an increase in the unwinding tensions.